don't expect all keys client-side
don't expect all keys client-side

server may be configured with a credential manager, so leave it to
runtime

concourse/concourse#291

Signed-off-by: Alex Suraci <a8053324ea51f0cc533cfe7b17f8bee3c920919f@pivotal.io>

concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse